The invention relates to a colour display tube comprising, in an evacuated envelope, a display screen and an electron gun system which has first means to generate three electron beams situated in one plane and second means to focus the electron beams on the display screen. The second means comprises at least two focusing electrodes which are common for the three electron beams and which have apertures for each electron beam. The centres of the apertures for the central electron beam are situated on the axis of the central electron beam. The centres of the apertures for the two outermost electron beams are situated at various distances from the axis of the central electron beam. The apertures for the three electron beams are defined by collars, starting with the side of the first focusing electrode remote from the first means, taken in the forward direction of the electron beams.
Such a colour display tube having a so-called integrated electron gun system in which a number of electrodes are constructed in common for the three electron beams is disclosed in Netherlands patent application 7809160, which corresponds to U.S. Pat. No. 4,291,257. In this known electron gun system an asymmetrical lens field is generated between the first and second means. By said asymmetrical lens field the two outermost electron beams are deflected in the direction of the central electron beam in such manner that the three electron beams converge on the display screen. As a result of the apertures for the outermost electron beams in the focusing electrodes, which apertures are staggered relative to each other a symmetrical focusing of the outermost electron beams is obtained. As a result of this a variation of the voltage at the focusing electrodes, and hence a variation of the strength of the focusing lens, has no influence on the convergence of the electron beams.
In the embodiment shown in FIG. 6 of the abovementioned patent application the electron beams are focused on the display screen by means of one single focusing lens field generated by two focusing electrodes.
Furthermore, integrated electron gun systems are known in which the electron beams are focused on the display screen by means of several focusing lens fields and in which three or more focusing electrodes are present which can be electrically interconnected in various manners. U.S. Pat. No. 4,063,340 discloses an integrated electron gun system having four focusing electrodes with which three focusing lens fields are generated. The focusing electrode which is last in the forward direction of the electron beams is at a high voltage potential. The first and third focusing electrodes are electrically interconnected and are at a potential which is approximately 40% of the high voltage potential. The second focusing electrode is at a potential which is approximately 25% of the high voltage potential. Furthermore, U.S. Pat. No. 3,863,091 discloses an electron gun system having four focusing electrodes in which the second and fourth focusing electrodes are electrically interconnected and are at a high voltage potential. The first and third focusing electrodes are electrically interconnected and are at a potential which is approximately 40% of the high voltage potential.
An electron gun system of the so-called unipotential type having three focusing electrodes of which the first and third focusing electrodes are electrically interconnected is known from U.S. Pat. No. 4,178,532. In such electron gun systems constructed with several focusing electrodes it is also desired for the convergence of the electron beams to be independent of the focusing of the electron beams. In integrated constructions, for that purpose, the apertures in the focusing electrodes for the outermost beams should be laterally staggered or shifted relative to each other analogous to the embodiment disclosed in Netherlands Patent Specification 7809160.
Assembling integrated electron gun systems is usually done by means of assembly pins connected in a jig which are threaded through the apertures of the electrodes. In integrated electron gun systems having apertures for the outermost electron beams in the focusing electrodes which are staggered relative to each other the pins for the apertures for the outermost electron beams should be provided with several eccentrics. However, such pins are difficult to manufacture and are hence expensive. Moreover, the size of the apertures in the various electrodes is restricted since after assembly of the electrodes it must be possible to remove the pins. The apertures in the various electrodes are preferably chosen to be as large as possible because the quality of the spot formed on the display screen by the electron beams improves as the diameters of the apertures become larger. Due to the tolerances in the dimensions of the apertures and the pitch between the apertures occurring during manufacture of the electrodes, the pins always have a slightly smaller diameter than the diameter of the apertures. As a result of this a certain positioning inaccuracy of the electrodes occurs which is largest when the pitch between the apertures is nominal.